This invention relates to a planar antenna and, in particularly, to a planar antenna for use in reading information related to a product from a μ chip which comprises a super miniature wireless IC (integrated circuit) chip embedded in the product as a product tag (an IC tag).
In prior art, in order to identify a product (an article), a bar code is pasted to the product. In addition, by reading the bar code by a bar code reader, information related to the produce is read.
In recent years, the super miniature wireless IC chip called the μ chip has been developed and the μ chip has been preliminarily embedded in the product as the produce tag (the IC tag). By transmitting and receiving an electric wave between the μ chip and an antenna, it is possible to read information related to the produce in question. The electric wave used has a frequency of 2.4 gigahertz (GHz).
As the antennas for transmitting/receiving the electric wave to/from the μ chip, those of various types are used. As one of such antennas, a planar antenna may be used which is disclosed in Japanese Unexamined Patent Publication Tokkai No. 2001-24428 or JP-A 2001-24428. According to JP-A 2001-24428, the planar antenna comprises an upper case, a planar antenna element disposed at backside of the upper case with first regular intervals, a ground plate disposed at second regular intervals from the planer antenna element, and feeding leads disposed between the planer antenna element and the ground plate.
In JP-A 2001-24428, in order to dispose the planar antenna element with the first regular intervals from the upper case, a plurality of small protrusions having first stepped parts are arranged at the central side of the backside of the upper case, and the planar antenna element having small holes at locations corresponding to the small protrusions is fixed to the upper case by fitting the small protrusions into the small holes. In addition, in order to dispose the ground plate with the second regular intervals from the planar antenna element, a plurality of large protrusions having second stepped parts are arranged near the peripheral of the backside of the upper case, and the ground plate having large holes at locations corresponding to the large protrusions is fixed to the upper case by fitting the large protrusions into the large holes.
In a conventional planar antenna, the plurality of small protrusions and the plurality of large protrusions are required to assemble the upper case, the planar antenna element, and the ground plate with intervals thereamong maintained at predetermined intervals. As a result, the conventional planar antenna is disadvantageous in that a lot of parts are required to assemble the conventional planar antenna.